In diverse industrial fields, attenuation of reflection and/or enhancement of light collection efficiency has been required and a number of relevant proposals have been made to this day.
By way of illustration, spectacles are required to possess the function of correction of vision, the function to protect the eye from foreign matter and unwanted light rays, and fashionableness. Moreover, constant demands exist for ever lighter and thinner lenses, while colored lenses are valued for their fashionableness. Under the circumstances and prompted by advances in the development of polymer materials having high refractive indices, high transparency, dyeability, etc., the demand for plastic lenses has by now surpassed the demand for glass lenses. The performance characteristics required of a plastic lens are high transparency, high refractive index, light dispersion, reduced weight, safety, dyeability, and moldability, among others. At the same time, decreases in the quantity of reflected light on the lens surface and increases in the transmissivity of the lens are also important parameters. Generally the reflected light on the surface of the lens on one side amounts to 3-4% and the tendency is that the larger the refractive index of a lens, the greater is the quantity of reflected light generated. The means employed today for reducing the quantity of reflected light consists in building up a plurality of thin metal films to extinguish reflected light through the mutual interference of reflected light rays among the layers and it is common practice to construct 3-7 antireflective layers. While this construction of antireflective layers is carried out using a vacuum vapor deposition equipment, a very large equipment is required for mass processing of lenses and this entails not only a high initial investment but also a high running cost both of which lead to appreciation of the lens cost.
The quantity of reflected light across the interface between a transparent, smooth-surfaced material and the air can be decreased by varying the mean index of refraction between said surface and air continuously. This objective can be accomplished by forming a heterogeneous layer on the surface of a transparent material. The principle of this heterogeneous anti-reflective technology is now discussed. Assuming, for instance, that the surface has irregularities such as those illustrated in FIG. 9, the refractive index [nf(x)], where x represents the direction of depth of the surface layer, can be expressed by the following equation (1). EQU nf(x)=ng.multidot.V(x)+n.sub.o (1-V(x) ) (1)
wherein ng represents the refractive index of glass, V(x) represents the volume of glass down to the depth x, n.sub.0 represents th e refractive index of air. Here, at the interface between air and the film and the interface between the film and the glass substrate, there is a discontinuous change in refractive index as illustrated in FIG. 10. writing the refractive indices at these points as n.sub.1 and n.sub.2, respectively, the index of reflection R of this layer can be expressed by the following equation. ##EQU1##
Assuming that, in this equation, n.sub.0 =1.0, n.sub.1 =1.1, n.sub.2 =1.477, ng=1.53, the lowest reflectance value is obtained when the surface irregularity value is 100 nm. However, it is quite difficult to form such microfine irregularities. To solve this problem, Japanese Kokai Patent Publication No. 2-175601 discloses a antireflective device and a method for its production, wherein ultrafine particles are immobilized in a filmy fashion directly on the surface of a transparent substrate to thereby satisfy both the requirements of low reflectance and high transmissivity. Here, ultrafine SiO.sub.2 particles with a small particle size variation are disposed in a monolayer on a glass substrate for improved transmission and a remarkable antireflective effect.
Ultrafine SiO.sub.2 particles can be immobilized in a monolayer on a glass substrate by the process which comprises dipping the glass substrate in a bath consisting of a solution containing ethyl silicate, ethanol, IPA, MEK, etc. as well as water and nitric acid for hydrolysis of ethyl silicate (S408, Asahi Glass Co.) and a 20 wt. % dispersion of SiO.sub.2 particles with a diameter of 120 nm in ethanol, raising the glass substrate from the bath in a perpendicular direction at a rate of 0.98 mm per second, allowing the volatile matter to vaporize, and baking the treated glass substrate in the air at 150.degree. C. for about 30 minutes to decompose the tetraethoxysilane. By this procedure, ultrafine particles of the decomposition product SiO.sub.2 are securely immobilized in a continuous, uniform thin layer.
FIG. 11 is a schematic view showing ultrafine particles of SiO.sub.2 as immobilized on a glass plate in the above manner. In the position indicated by the symbol A in the drawing, the refractive index is equal to the refractive index of air, no, which is taken as 1. In position B, the refractive index is equal to the refractive index of the ultrafine powder 4, i.e. n=1.48. The refractive index of the portion surrounded by these positions A and B can be calculated by means of the above equation (1) and the refractive indices in intermediate positions are considered to vary in proportion with the percentage of the volume occupied by SiO.sub.2 in the total volume of the immaginary small plate shown in the drawing. Writing the refractive index in position C which is situated slightly inward of A (on the glass plate side) as n.sub.1, the refractive index at B where the volume occupied by SiO.sub.2 particles in said immaginary small plate accounts for approximately 100% as ng, and the refractive index in position D which is situated slightly outward of B as n.sub.2, the conditions under which the surface reflectance R of the glass plate becomes minimal are given as follows. ##EQU2##
It is apparent that a non-reflective state is obtained when the condition of ng-n.sub.2 /n.sub.1 is satisfied. Here, the value of n.sub.2 /n.sub.1 is dependent on the shape of irregularities. Since n.sub.1 and n.sub.2 are values determined according to the percentage volume of SiO.sub.2 within the immaginary plate, they might appear to be unrelated to the diameter of ultrafine particles but as experimentally demonstrated, production problems are encountered if the particle diameter is smaller than about 30 nm. Thus, if the particle diameter is too small, the irregularities are too much smoothened out to lose the ability to inhibit reflection of light. On the other hand, if the diameter is larger than about 300 nm, opacification takes place to reduce the degree of transparency. Therefore, as mentioned above, the optimum antireflective performance can be obtained when the thickness of said monolayer of ultrafine particles is approximately 100 nm.
There is no serious problem with the immobilization of SiO.sub.2 in a monolayer on a glass substrate but when the same ultrafine particles are to be immobilized on a clear resin plate, a binder exhibiting strong adhesion to the common transparent substrate such as an acrylic board or a polycarbonate board is not available. No suitable binder is available, either, for lens and other optical materials such as CR-39 and urethane resin. Therefore, the present applicant attempted a direct transfer of a microgranular monolayer surface onto a transparent resin substrate for the formation of ultrafine irregularities on the resin surface and disclosed the result in Japanese Patent Application No. 5-330768.
The above technology was more or less satisfactory in that an original microgranular surface can be exactly transferred but when the transfer process was repeated, such troubles as dislodging of ultrafine particles, difficulty in mold release, and non-uniformity of the transfer surface occurred, so that the need for improvement was felt. However, if the quantity of the binder is increased to prevent dislodging of particles, the ultrafine particles tend to be embedded in the binder to detract from the function of achieving a continuous gradation of refractive index. Moreover, in order to insure an exact transfer of ultrafine particles for faithful reproduction of the radii of curvature of the particles, the quantity of the binder should be as small as possible. Regarding the difficulty in mold release, the resin finds its way into the intergrain spaces to form anchors which require a large releasing force and may cause breakage of the matrix. Therefore, a novel matrix free from such problems has been demanded.
The present invention has for its object to provide a lens with reduced surface reflectance as manufactured using a novel matrix improved in the above aspects for the transfer of a microgranular monolayer surface.
The present invention is further addressed to improvement of the light collection efficiency of solar cells.
Devices adapted to generate electricity in response to excitation by light have been utilized as sensors in illuminometers and for finder control in cameras, among other applications, and highly efficient optical transducers such as silicon semiconductors have appeared in recent years. As to such semiconductor materials, silicon monocrystals and polycrystals are generally employed and the demand for amorphous silicon, in particular, is rapidly growing because of its advantages in production methods and costs. Meanwhile, in the field of solar batteries, the absorption wave-length band varies with the physical properties of semiconductor materials constituting the solar cell. Thus, whereas crystalline silicon generally has a sensitivity peak in the infrared region from about 800 nm to about 1000 nm, amorphous silicon has a sensitivity peak in the visible region of the spectrum. Moreover, the light-receiving surface of a semiconductor material is metal-polished, the reflection of light is large, with reflected light accounting for about 35% of incident light. Therefore, it is common practice to deposit 2-3 thin metal layers on the surface by the vacuum vapor deposition technique so as to suppress the proportion of reflected light to less than 10%.
As to means for reducing the reflection of light, several methods are available in addition to the above method in which the VVD films are used for exploitation of interferences of light. Thus, the method comprising coating with a solution of some fluorine-containing low-refractive-index resin or a dispersion of a low-refractive-index substance such as MgF.sub.2 so as to reduce the quantity of reflected light at the interface with air can be mentioned by way of example. In such methods, the reflection characteristics vary with the thickness of the coat and the layer structure but, as general trends, the characteristics shown in FIG. 23 are obtained. Thus, Curve R5 represents the reflection characteristic of the material coated with a solution of fluorine-containing low-refractive-index resin, indicating a reflectance of 0.8-1.0% around 550 nm and a reflectance of 2-3% for incident light with wave-lengths of 400 nm and 800 nm. When the monolayer film is composed of MgF.sub.2 (refractive index 1.38), Curve R3 is obtained. In the case of SiO.sub.2 (refractive index 1.46), Curve R2 and, in the absence of a layer, Curve R1 are respectively obtained. When a multi-layer film is used as in the case of a lens, the reflection characteristic of Curve R4 is obtained, indicating a lowest reflectance of 0.2% around 450 nm and 650 nm but about 1.5% at 500 nm. While the conventional antireflective layer is thus constructed by techniques suited to applications, it is desirable to provide a material which would show low refractive index constantly over the range of 400 nm to 800 nm. Moreover, the construction of a multi-layer film by the vacuum vapor deposition technique requires costly equipment and is time-consuming, thus leading to high production cost.
When light passes through a substance, it is partially reflected by the surface of the substance. To prevent this reflection, there is a method in which the refractive indices of two adjoining substances are varied continuously. This method comprises immobilizing ultrafine particles with a diameter of 30 nm-600 nm in a single parallel array and the standard procedure is disclosed in Japanese Kokai Patent Publication No. 2-175601. Moreover, the present applicant proposed a technique for providing a transparent member with an antireflective function which comprises transferring a surface constituted of ultrafine particles to the transparent member in Japanese Patent Application No. 5-330768 and, as a typical application of the technique, disclosed a case of application to the CD in Japanese Patent Application No. 7-29222.
It is an object of the present invention to apply the above technology to enhancement of the light collection efficiency and expansion of the light-receiving area of a solar cell.
It is a further object of the invention to prevent the surface reflection of an optomagnetic recording medium.
The optomagnetic recording medium is generally available in the form of a disk which is easily stowable. Therefore, the optomagnetic recording medium is commonly known as an optical disk, and according to uses, is classified into a read-only disk, a writable disk, a rewritable disk, and so on. Data are transcribed or written on the recording surface of the medium and the record is read out with laser light for reproduction.
Unlike solar light, the laser light used for writing and reading of information on an optomagnetic recording medium is a light of a single wavelength band and no mass with good linearity and high collection efficiency. The wavelength of laser light varies with different light sources but for optomagnetic recording and playback, laser light within the wavelength range of 633-830 nm is frequently employed. The wavelengths of such laser emissions are close to the upper limit of the visible region of the spectrum.
When a laser light passes through a transparent body, it is refracted according to the refractive index of the particular transparent body, while it is reflected by a mirror surface. An optical recording medium records signals according to the lengths of pits and the depth of the pits is usually set to about 0.1 .mu.m. In the readout of signals, the laser light spot is projected to the pit and surrounding area. By utilizing the phenomenon that the quantity of reflected light from the pit is decreased by the interference of reflected light rays due to the difference in the light path, the presence or absence of such reflected light rays and the length of the pit are converted to a digital signal.
It is not true that when a laser light is incident on a transparent body, all the laser light passes through the transparent body but rather a portion of the light is reflected by its surface. The reflectance can be calculated on the basis of the intrinsic refractive index of the very substance by means of the equation of Fresnel. EQU R={(ng-n.sub.0)/(ng+n.sub.0)}.sup.2
The reflectance values of polycarbonate and acrylic resins, which are generally used for optical disks, as calculated by the above equation are 5.2% and 3.9%, respectively, for one side. In the optical disks used today, the transparent members are not provided with means for preventing surface reflectance. In the above equation, ng represents the refractive index of the substrate material and no represents the refractive index of air.
For the purpose of decreasing the surface reflectance of a transparent member located on the readout side of an optical disk, the method utilizing the interference of light, for example the procedure which comprises forming a thin film of metal by vacuum vapor deposition for exploiting the interference of light, is not suitable. Thus, the method which comprises varying the refractive index continuously to decrease reflected light is more advantageous in that the phase of light is not disturbed.
In view of this fact, an object of this invention is to provide a system in which a layer varying continuously in refractive index is disposed on the readout side to attenuate reflection.
It is another object of the present invention to provide a photosensitive material having an antireflective function.
In regard of information recording, the technique of digital recording with the use of magnetism or laser light is meeting the contemporary needs because of its instancy. However, photography which provides picture images with greater certainty in an analog fashion is still assuming a solid position. As a recording medium, photography uses a photosensitive material which can be prepared by dispersing crystal grains of a silver halide, which is a photosensitive substance in an aqueous solution of gelatin, coating the resulting photographic emulsion in a thickness of about 10-30 .mu.m on a plastic film or a sheet of glass or paper, and drying the coat. The coating amount of silver halide in this photosensitive material is about 1 mg/cm.sup.2. The silver halide may for example be silver bromide, silver chloride or silver iodide.
A schematic cross-section view of a negative film for general photography using a plastic film as a support, which is an example of said photosensitive material, is presented in FIG. 44. The reference numeral 430 represents a protective layer which protects the emulsion grains from mechanical injuries associated with scratching or abrasion. The thickness of the protective layer is limited to about 1 .mu.m. The reference numeral 431 represents a photographic emulsion layer which comprises a dispersion of crystal grains of a silver halide in an aqueous solution of gelatin and its dry thickness is usually about 15-25 .mu.m. Indicated at 432 is a support which may be a plastic film such as polyethylene terephthalate film or cellulose triacetate film. Though it varies with different uses, the thickness of the emulsion layer is 85-200 .mu.m. The reference numeral 433 represents a backing layer which is a hard gelatin layer more or less as flexible as the photographic emulsion layer 431. The function of this backing layer is to prevent curling of the photosensitive material.
Another example of the photosensitive material is a sensitive paper (photographic printing paper). FIG. 45 is a schematic cross-section view of the sensitive paper. The reference numeral 434 represents a protective layer predominantly made of gelatin, the thickness of which is about 1 .mu.m. The reference numeral 435 represents an emulsion layer, which comprises a dispersion of crystal grains of a silver halide in an aqueous solution of gelatin. The thickness of this layer is fairly thick, i.e. 5-12 .mu.m. The reference numeral 436 represents a paper support. This support 436 is sturdy enough to resist the alkaline developer and acidic fixative and withstand prolonged water rinsing, and its thickness is about 0.04-0.3 mm and is usually classified into the thick, medium, and thin grades. A baryta layer 437 is disposed on one side of the support 436. This baryta layer 437 is a layer formed from a mixture of barium sulfate crystals and gelatin on the paper stock for increasing the reflectance and gloss of paper and preventing the emulsion from seeping into the fibrous layer of paper.
As described above, the surface layer of a photo-sensitive material is protected with a thin film made predominantly of gelatin. Since exposure light of necessity passes through this protective layer, reflected light is generated at the interface with air to reduce the quantity of transmitted light. As a general procedure for reducing this reflected light, the above-mentioned technique comprising vapor-deposition of a metal in multiple layers is available. However, in the case of a photosensitive material, chemicals should diffuse through the gelatin layer and, therefore, a metal film is not suited. As mentioned hereinbefore, there also is a method which comprises varying the index of refraction continuously to reduce the reflection of light at the interface with air. Thus, in this method, ultrafine particles (particle diameters 30-600 nm) of SiO.sub.2 are immobilized in a single parallel array. This technique can be basically applied to the surface of a photosensitive material but the immobilization of particles calls for a certain degree of bond strength and any adhesive of the gelatin series cannot satisfy this bond strength requirement. Moreover, other kinds of adhesives may detract from the diffusion of chemicals through the gelatin layer.
Accomplished with the above facts taken into consideration, the present invention has as a further object to provide a photographic light-sensitive material equipped with an antireflective function and yet fully retaining the properties of gelatin constituting the protective layer of the light-sensitive material.
The present invention is further directed to a method for preventing the reflection of irradiation light in the light path in the fabrication of an electronic circuit, to an associated apparatus, and to associated products.
For focussing an image pattern of an electronic circuit on a photoresist in the manufacture of an IC or LSI, there are available a variety of methods for light exposure, such as contact, close-up, life-size, and reduction methods. The fundamental principle of such methods comprises irradiating a photomask having a pattern based on a circuit design with light from a light source and focussing the pattern on a photoresist through a predetermined optical system. While the interposition of a pattern creates areas allowing the passage of exposure light and shielded areas, the transmitted light is preferably a beam of parallel rays not containing interference light and diffracted light. For this reason, a film or a quartz glass sheet is used as the substrate of a photomask but at least 4% of exposure light is reflected at the interface with air so that the quantity of transmitted light is as much decreased. In addition, when transmitted light passes through different substances, reflected light is scattered in random directions to create interference light. Therefore, an antireflective layer is required at least between the transparent support and the light-shielding layer for the formation of a pattern.
It is a further object of the present invention to provide an expedient method and apparatus for imparting an antireflective function to a light-transparent material such as a photomask or the interface of substances constituting a laminate such as a photo-resist in the fabrication of an electronic circuit by optical means.